In our toys, we introduce the X sound using words with the most common sound it represents (ks). This usually occurs at the end of a word (box, fox) or in the middle of a word (exit), which is why the audio in our toys does not state that the word "starts" with a particular letter.
While there are some words that start with the NAME of the letter (x-ray) and there are words that start with a less common sound for x (xylophone), there are no words that start with the most common SOUND for X (ks). This is why Lettersaurus uses a word with the X sound in the middle.